Cover for a chair base



FIG. 1 is a top perspective view of a cover for a chair base showing my new design;

FIG. 2 is a bottom perspective view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom plan view thereof;

FIG. 5 is a side view thereof;

FIG. 6 is another side view thereof. 

The ornamental design for a cover for a chair base, substantially as shown and described. 